Fix for IE9 html/htmlcollection test with Safe HTML

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 0ca2729f616500cbe0abcc55aad2cd5a1c3d42b4..bd228d2f4934bc59f8a92e335578632d2ecdd7c3 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -33809,7 +33809,8 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
         var keys = attrs.keys.toList();
         for (var i = attrs.length - 1; i >= 0; --i) {
           var name = keys[i];
-          if (!validator.allowsAttribute(element, name, attrs[name])) {
+          if (!validator.allowsAttribute(element, name.toLowerCase(),
+              attrs[name])) {
             attrs.remove(name);
           }
         }